The invention relates to a method to protect cryogenic heat exchangers when the products from high temperature conversion of mercury-containing hydrocarbon feeds are cooled. The invention relates also to a method for reducing mercury to negligible levels in thermal cracking product steams.
Thermal cracking is a process in which heat is used to crack hydrocarbon feedstock. Steam cracking is the thermal cracking and reforming of hydrocarbon feedstocks with steam, to light olefins, such as ethylene, propylene, butenes and butadienes, generally carried out at low pressure and high temperature for short residence times. Steam is used as a diluent to achieve a low hydrocarbon partial pressure resulting in high product yield.
After a hydrocarbon feedstock has been subjected to high temperature cracking conditions, the product effluent may be cooled, dried and liquified in a cryogenic heat exchanger. Heat exchangers are often made of aluminum which can form an amalgam with mercury resulting in corrosion and cracking of the heat exchanger. When the feedstock contains mercury or mercury compounds, the resulting product effluent is contaminated with mercury. If it is not removed, the mercury damages the aluminum components of the heat exchanger.
The amount of mercury in hydrocarbon feeds varies with the type and geological origin of feeds. Liquid condensates from natural gas fields in particular contain significant levels of mercury.
A number of methods have been developed for removing mercury from gases and liquids using compounds supported by an adsorbent mass. Methods of this type are described in U.S. Pat. Nos. 4,094,777, 4,101,631, 4,474,896, 4,709,118, 4,892,567, 4,909,926 and French Patent No. 75/15579.
U.S. Pat. No. 4,094,777 and the French Patent employ a metal or metal compound supported by an adsorbent mass such as alumina or silica-alumina.
U.S. Pat. No. 4,101,631 describes the removal of mercury vapor by contacting a gas stream at -40.degree. to 100.degree. C. with zeolitic molecular sieves containing elemental sulfur.
U.S. Pat. No. 4,474,896 discloses the use of polysulfide-containing adsorbent compositions to adsorb mercury from gaseous or liquid streams.
U.S. Pat. No. 4,709,118 describes removing mercury from hydrocarbon liquids or gas by contacting with a reduced milled mixture of bismuth or tin oxide and silica or alumina base.
U.S. Pat. No. 4,892,567 describes a method for the simultaneous removal of mercury and water from a hydrocarbon fluid by contacting the fluid with zeolite A containing elemental silver or gold on its surface. Among the above-listed patents, only U.S. Pat. No. 4,909,926 describes chemisorption of mercury in condensate at high temperatures. In the method described in U.S. Pat. No. 4,909,926, the adsorption temperature is kept high to discourage adsorption of heavy compounds and improve adsorption of mercury. The optimum adsorption temperature depends on the endpoint of the condensate. Although the high temperature adsorption is effective in removing mercury from hydrocarbon oil, in a high temperature cracking process, even very small residual amounts of mercury remaining after adsorption treatment are converted to mercury vapor which is potentially damaging to the environment and also highly capable of damaging aluminum heat exchangers. U.S. Pat. Nos. 4,892,567 and 4,909,926 are incorporated by reference herein in their entireties.
Although various methods for removing mercury from gases and liquids have been described, none suggests providing a backup method to insure that mercury in product steams is reduced to the most minimal levels to avoid damage to cryogenic heat exchangers and to the environment.
Accordingly it is an object of the invention to supply a method which provides a good margin of protection against the incursion of mercury to the aluminum heat exchanger in high temperature hydrocarbon conversion processes.
It is a further object of the invention to supply a method for reducing mercury to extremely low levels in the product streams resulting from the thermal cracking.